1. Field of the Invention
This invention relates to an X-ray image-processing apparatus, such as, particularly, a digital radiographic device.
2. Description of the Prior Art
In the X-ray image-processing apparatus, such as the digital radio-graphic means, an X-ray image of human tissue (hereinafter called a "mask image"), for example, is photographed and then, a contrast medium is immediately injected into a blood vessel of the human tissue. Thus, an X-ray image (hereinafter called a "contrast image") is photographed, immediately after the injection of the contrast medium. The photographed X-ray image is converted into image signals, by means of digital processing. The image signals of the mask image are digitally subtracted from those of the contrast image, thereby causing generation of subtraction image signals. A subtraction image obtained from the subtraction image signals, for example, i.e., only an X-ray image of a blood vessel, is displayed on a display unit and is adapted for a vessel diagnosis.
A system for obtaining a digital subtraction image includes a system for photographing a digital image prior to the subtraction processing. Known systems for obtaining a digital subtraction image include, for example, an image intensifier and a TV camera system, as disclosed in U.S. Pat. No. 3,859,527 (George W. Luckey), an imaging plate system, as disclosed in U.S. Pat. No. 4,485,304 (Teraoka et al). In general, the imaging plate system has the following advantages in comparison with the image intensifier and TV camera system:
The imaging plate system has a very high capability to decompose space in comparison with the system employing the image intensifier and the TV camera. Further, whereas the photographing scope of the system employed in the image intensifier and TV camera is limited to the light-receiving surface of the image intensifier, that of the system employing the imaging plate can be used for the entire surface of the imaging plate. Since it is easy to enlarge the imaging plate, the imaging plate system can thus have a larger photographing scope than the system employing the image intensifier and TV camera.
An X-ray image means for photographing a digital subtraction image, employing the imaging plate system which has such advantages, are disclosed, for example, in U.S. Pat. No. 4,485,304 (M. Teraoka et al), and Japanese Laid Open Patent Publication No. 59-7250. The conventional X-ray image means as disclosed in those publications has an imaging plate-changer, to move the imaging plate to a photographing area, thereby positioning the imaging plate to be opposed to the X-ray tube. The object to be examined, for example, the human body, is laid therebetween. Under such circumstances, a mask image is photographed, for the purposes of examination, and, in lieu of the imaging plate on which the mask image has been photographed, a second imaging plate is moved into position, and a contrast image of the object to be examined, is photographed, following the injection of the contrast medium. These two imaging plates are moved, one after another, to an image input means, by the imaging plate-changer, and their respective surfaces are scanned by laser light. Thus, the image area photographed on the imaging plates is emitted successively, so that any emitted information is detected by a detector and is converted into electric signals. After the image information of the mask image and contrast image has been converted into digital signals, a subtraction image is obtained by means of preferred digital subtraction processing. Further, this digital image is again converted into analog signals, and the resulting image is displayed on a CRT monitor or the like.
Accordingly, after the contrast image has been photographed, photography must be performed in a short time interval prior to the injection of the contrast medium. In the method using a conventional imaging plate-changer, the plate-changer is required to move two imaging plates, one after another, at high speed, instantaneously determine the position of their photography area, and stop them in the correct photography position. Further, the imaging plate-changer must be provided with a high-speed servo system and a high-precision positioning-control system, with the result that its construction becomes complicated and its manufacturing cost consequently becomes expensive.